1. Field of the Invention
The present invention relates to a projection type liquid crystal display device. More particularly, it relates to such a device which is capable of writing and erasing an image only by application of voltage and of displaying the information from a remote place and which is compact and produces a bright display image at a projection area even in bright surroundings. Such a display device is suitable to be used for conferences, education, and public notices such as in theaters, music halls, department stores, railway stations, airport lobbies, etc.
2. Description of the Related Art
Heretofore, projection type cathode ray tubes (CRT), laser light writing projection type liquid crystal display devices, and CRT writing projection type liquid crystal display devices have been used as projection type display devices for relatively bright rooms (see IBM. J. Res. Develop. Vol. 26, No. 2, pp. 177-185 (1982); Hughes, SID 79 Digest pp. 22-23 (1979). These devices are of the scanning type. Also known are overhead projectors (OHP) which project an original directly onto a screen.
Projection type CRT's have projection devices built in so that an image formed on a CRT display surface is projected onto a screen. Laser light writing projection type liquid crystal display devices project onto a screen an image formed in a liquid crystal panel by changing alignment states thereof by heat from the laser light. CRT writing projection type liquid crystal display devices comprise a liquid crystal panel comprising a laminate of a liquid crystal layer and a photoconductive layer between electrodes. A CRT irradiates the photoconductive layer with an image like light to change the voltage applied to the liquid crystal layer so that an image corresponding to the irradiated light is formed in the liquid crystal layer, which image is projected onto a screen.
OHP's simply project an image formed on a transparent or translucent sheet with an opaque colorant onto a screen by white light.
Projection type CRT's are essentially a light emission type and therefore cannot use a light source for projection having a large quantity of light, such as a halogen lamp, resulting in a dark projected image on a screen. Since it projects an image formed on an image face of a CRT, the hole device becomes large so that it ordinarily has to be used in a fixed place, and is not portable.
Laser light writing projection type liquid crystal display devices have advantages that light sources having a large quantity of light can be used for projection and both reflection and transmission type projections can be done so that the projected image is bright. These devices however use writing laser light providing heat to a liquid crystal to thermally cause phase transition and form an image, by which a display is attained. This mechanism necessitates large electric power applied to the liquid crystal. Further, because the heat necessary for causing the phase transition of a liquid crystal is provided by light, the output of laser light must also be large and a light source such as an argon laser is necessary, which makes it impossible to make the device compact.
CRT writing projection type liquid crystal display devices have a large light source, though smaller than that of the above laser light writing devices. Since an image is transferred from an faceplane of a CRT to a liquid crystal panel, the faceplane of the CRT or the original image cannot be made large. As a result, the image must be magnified by providing a large distance from the liquid crystal panel to a projection screen and therefore, the brightness of the image on the screen is not sufficient.
In contrast, projection type liquid crystal display devices in which writing is conducted only by application of voltage enable remarkable compactness and light weight and promise reduction of production costs (see, for example, Japanese Unexamined Utility Model Publication (Kokai) Nos. 59-131740, 59-151235, and 59-151236 and Japanese Unexamined Patent Publication (Kokai) Nos. 59-19924 and 59-37533). However, only dynamic scattering mode (DSM) or twisted nematic (TN) type liquid crystal display devices are conventionally used. They cannot provide a display having high information contents and do not have sufficient information for use in conferences, etc. Although a TN type liquid crystal display device with an about 640.times.200 dot display has become available recently, TN type devices must have two polarizers, which remarkably reduces the quantity of light transmitted through the liquid crystal panel and makes a projected image dark.
OHP's have been widely used in conferences of small numbers of participants since they have advantages of light weight, compactness and low cost, can display an original image directly on a screen, and allow writing on the original sheet in situ with an oil pen. Further, the original sheet usually is of an A4 size, so not that large a magnification is necessary and the image on the screen is bright enough to see even in a bright room. However, to display with an OHP, preparation of OHP sheets is necessary and fine lettering is required depending on the nature of the conference, etc. OHP sheets once made cannot be modified during display. If there are a large number of OHP sheets, carrying them and changing sheets in the OHP are troublesome.
Recently "electronic conference systems" have been proposed and sold, enabling conferences between remote places through telephone cables or radio systems. These use devices capable of simultaneous display in each conference room, that is, display devices which can display, at all conference rooms, an image that is under discussion in any one conference room in real time by means of some image transmitting medium. For these applications, OHP's cannot be used. Projection type CRT's can be used, but have a drawback of dark images. Moreover, if a projection type CRT or a laser light writing projection type LCD is used, special conference rooms must be prepared, since the devices are large in size and heavy in weight.
Thus, projection type display devices which have the facility and universality of OHP's and are capable of transferring and receiving images by means of telephone lines, etc. are desired.
Further, if compact projection type display devices capable of providing bright, large capacity displays become available, they would be useful in many applications, e.g., as notice boards at airport lobbies, railway stations, theaters, etc. and as educational instruments.